Flutter：应该只有一项具有 [DropdownButton] 的值

我正在尝试创建一个下拉按钮我的数据库中的列表然后我把名单传给我的dropdownButton 一切正常当我从中选择一个元素时我收到以下错误：

There should be exactly one item with [DropdownButton]'s value: Instance of 'Tag'. 
Either zero or 2 or more [DropdownMenuItem]s were detected with the same value
'package:flutter/src/material/dropdown.dart':
Failed assertion: line 805 pos 15: 'items == null || items.isEmpty || value == null ||
          items.where((DropdownMenuItem item) {
            return item.value == value;
          }).length == 1'

我试过设置DropdownButton值为null它起作用了，但我无法查看所选元素.

这是我的代码：

FutureBuilder(
    future: _tagDatabaseHelper.getTagList(),
    builder: (BuildContext context, AsyncSnapshot snapshot) {
      if (!snapshot.hasData) {
        return Center(
          child: CircularProgressIndicator(),
        );
      }
      return ListView(
        children: [
          SizedBox(
            height: MediaQuery.of(context).size.height * 0.2,
          ),
          Container(
            margin: EdgeInsets.symmetric(
                horizontal: MediaQuery.of(context).size.width * 0.07),
            child: Theme(
              data: ThemeData(canvasColor: Color(0xFF525A71)),
              child: DropdownButton(
                value: _selectedTag,
                isExpanded: true,
                icon: Icon(
                  Icons.arrow_drop_down,
                  size: 24,
                ),
                hint: Text(
                  "Select tags",
                  style: TextStyle(color: Color(0xFF9F9F9F)),
                ),
                onChanged: (value) {
                  setState(() {
                    _selectedTag = value;
                  });
                },
                items: snapshot.data.map((Tag tag) {
                  return DropdownMenuItem(
                    value: tag,
                    child: Text(
                      tag.tagTitle,
                      style: TextStyle(color: Colors.white),
                    ),
                  );
                }).toList(),
                value: _selectedTag,
              ),
            ),
          ),

我用过futureBuilder公司到从数据库获取我的列表.